1. Field of the Invention
The present invention relates to a control device that controls mold clamping in an injection molding machine. More particularly, the present invention relates to a mold clamping control device capable of accurately controlling mold clamping forces that is exerted on a mold during injection molding, to thereby reduce defective appearance of ultra-precision molded articles.
2. Description of the Related Art
A typical injection molding machine with a toggle type mold clamping device (mold clamping mechanism) controls a position of a so-called “cross-head” to generate a mold clamping force. The mold clamping force is exerted on a mold. The mold includes a fixed mold segment and a movable mold segment. During the operation, the mold clamping force should be controlled for respective steps of the injection molding stage. An example of a conventional mold clamping force control device is disclosed in Japanese Patent Laid-Open No. H10-119100. This device employs a strain gauge mounted onto a member such as a tie bar that receives the stress caused by the mold clamping force. The amount of strain indicated by the strain gauge is detected as the mold clamping force. The mold clamping force is controlled on the basis of the detected mold clamping force.
Upon injection molding, the movable mold segment closely contacts to the fixed mold segment in order to close the mold. The mold clamping force is exerted on the mold before charging a mold cavity with a molten resin. For this purpose, a mold clamping motor is controlled in accordance with a detection value supplied from a position detector so that the actual position of a movable platen coincides with a predetermined position for the movable platen. Likewise, the mold clamping motor is controlled in accordance with a detection value supplied from a mold clamping force detector so that the actual mold clamping force becomes equal to a predetermined mold clamping force.
The molded article which is being formed by the injection molding process may sometimes be an article requiring very high precision, such as a molded article with a complicated configuration, or a thin molded article. For the production of such high precision molded articles, the movable platen is shifted and/or the mold clamping force is changed to improve the quality of the final molded articles. This shifting or change of force is sometimes referred to as “toggling”. These operations are made during the time from the beginning of the charge to a dwelling/cooling stage.
To this end, the conventional toggle type mold clamping device either shifts the movable platen (i.e., movable platen position control) or changes the mold clamping force (i.e., mold clamping force control) during the time from the beginning of the charge to the dwelling/cooling stage. However, these two factors have different functions. The movable platen position control is effective for providing stability in thickness and weight of the final molded articles. The mold clamping force control, on the other hand, is effective for the improvements of defective appearance and optical properties. Nevertheless, the conventional toggle type mold clamping device performs either the movable platen position control or the mold clamping force control at a time. This means that only advantages of either one of the controls are obtained. No benefits or advantages of the two controls are achieved. Such conventional control of the mold clamping force is less accurate. Thus, it is difficult to use it for the ultra-precision molding.